Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the U.S. with an annual incidence of 5 million cases. Studies have been in progress to define the clinical spectrum of chlamydial infection, to develop improved diagnostic assays, and to examine the pathogenesis of chlamydial infections. In screening 4,000 patients attending STD clinics, we have demonstrated that molecular amplification assays, including polymerase chain reaction (PCR) and ligase chain reaction (LCR) have markedly improved the sensitivity of detection of C. trachomatis. In screening urine samples by PCR or LCR, the prevalence of chlamydia infection was as high as 28% in school-based adolescent clinics compared to 15% by genital culture. In transmission studies of 106 sexual partnerships with chlamydial infection, 52% were concordantly PCR positive. In contrast to culture results, there was no significant difference in male-to-female vs. female-to-male transmission, which was estimated to be 68%. Genetic sequence analysis of amplified chlamydial DNA from these partner pairs confirmed the presence of the same serovar, further supporting the accuracy of PCR. Similar results have been obtained by LCR for the detection of Neisseria gonorrhoeae. We developed a new serologic technique for the measurement of specific antibodies to the major outer membrane protein (MOMP) referred to as a transcription-translation radioimmunoassay (TT-RIA). Use of this assay demonstrated a serologic rise in specific anti-MOMP antibodies in MOMP- vaccinated primates, which directly correlated with decreased infectious load. Following the development of PCR for C. pneumoniae, we have identified C. pneumoniae in 8% of 700 patients with respiratory disease. Antimicrobial susceptibility studies have demonstrated the potential efficacy for azithromycin and clarithromycin for treatment of this pathogen. Although we have been able to demonstrate in vitro replication of C. pneumoniae in human pulmonary macrophages, aortic endothelial cells, and aortic artery smooth muscle cells, we have not been able to demonstrate by culture or PCR C. pneumoniae in atheromas of selected patients with coronary heart disease. Additional studies are planned to further address the role of C. pneumoniae in respiratory disease and atherosclerotic heart disease.